Did you mean the "server connection" ?

yes

I am assuming you're speaking about a Dedicated Server.

First Scenario: You're clicking the wrong tab. After you choose your character, you are defaulted to the "Start Game" tab. Make sure you click the "Join Game" tab at the top.

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Second Scenario: If the server is located at either your son's house, or your house, and one of you play locally and the other remotely, then the one connecting remotely will use the server's external IP, while the one connecting locally will use the internal IP.

In this case, the one hosting the server, must create the proper UDP and TCP port forwards for the port range 2456 to 2458, and make an exception in their Firewall/Anti-virus for the Valheim server app. Whoever is connecting remotely will most likely use the following: External-IP-Address:2457

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Let's work with these two, until you find neither of these work.

PS: you can hover your mouse cursor over the upside down chevron icon and click "Edit" to make changes to your post contents.